JP2006-147933A (hereinafter, referred to as “Document 1”) discloses a light-emitting diode lighting device as a conventional example. This conventional example includes a rectifier circuit, a light-emitting diode circuit, a current limiting resistor, and a lighting control circuit. The rectifier circuit is configured to full-wave rectify a sinusoidal AC voltage supplied from an AC power source of which effective value is 100 V to produce a pulsating voltage. The light-emitting diode circuit includes multiple light-emitting diodes which are connected in series with their polarities in the same direction. In the light-emitting diode circuit, the multiple light-emitting diodes are series-connected so that each anode of the light-emitting diode is directed to a positive side of the rectifier circuit and each cathode thereof is directed to a ground. The multiple light-emitting diodes of the light-emitting diode circuit is divided into multiple groups (first to sixth diode circuits) each of which includes a certain number of light-emitting diodes connected in series. The current limiting resistor is connected between the rectifier circuit and the light-emitting diode circuit to limit a current flowing through the light-emitting diode circuit.
The lighting control circuit includes first to sixth drive-switches which are individually connected in series with respect to the first to sixth diode circuits. The lighting control circuit is configured to turn on and off the first to sixth drive switch units in turn in accordance with an instantaneous value of the pulsating voltage from the rectifier circuit to thereby light the first to sixth diode circuits in stages.
Thus, the conventional example described in Document 1 changes (increases or decreases) the number of lighted light-emitting diodes (i.e., the number of light emitting diodes that are turned on) in accordance with the instantaneous value of the pulsating voltage. Accordingly, the multiple light-emitting diodes connected in series can be efficiently lit with a simple circuit structure.
In the conventional example described in Document 1, however, the light amount thereof varies according to the number of lighted light-emitting diodes. Further, in the conventional example in Document 1, there exists a period in which no light-emitting diode is lit. Thus, the flicker of the emitted light thereof may become a problem in some situations. Specifically, in video images taken by a video camera, the brightness of the images would be expected not to fluctuate frequently.
Besides, in a structure where a smoothing capacitor for smoothing the pulsating voltage is connected between output terminals of a rectifier circuit, there is a problem in that an excessive rush current flows through the smoothing capacitor upon the AC power being switched on.